In internal combustion engines, the fuel vapor canister primarily adsorbs refueling vapors, as refueling and diurnal vapors are sealed within the fuel tank by a fuel tank isolation valve. An air intake system hydrocarbon (AIS HC) trap may capture hydrocarbons emitted by leaky injectors and/or from fuel that may puddle in the engine intake. The AIS HC trap may also capture uncombusted fuel that is trapped within the engine cylinders themselves. An AIS HC trap is required for vehicles to be classified as practically zero emissions vehicles (PZEVs).
The contents of the AIS HC trap may be purged to engine intake during engine operation by opening an intake throttle plate, thus directing fresh air through the trap and desorbing bound hydrocarbons for combustion. However, hybrid vehicles may operate for prolonged periods without combusting fuel, thus limiting opportunities to purge the fuel vapor canister and AIS HC trap for combustion. Prolonged periods without AIS HC trap purge may cause degradation of the AIS HC trap. Further, liquid inhalation may damage the adsorbent material present in the HC trap.
One example approach for periodically or opportunistically purging the AIS HC trap is shown by Dudar in U.S. Patent Application No. 20170234246. Therein, the AIS HC trap is purged to a fuel vapor canister during an engine non-combusting condition by reverse rotating the engine via an electric motor. Reverse rotation of the engine causes atmospheric air to enter an intake of the engine via an exhaust of the engine, desorbing hydrocarbons bound to the air intake system hydrocarbon trap.
However, the inventors herein have recognized potential issues with such systems. As one example, as the AIS HC trap is purged, degradation of the HC trap is not diagnosed. Operating an engine with a degraded HC trap and purging a degraded HC trap may result in an increase in bleed emissions.
In one example, the issues described above may be addressed by an engine method comprising: during unfueled cranking of an engine, testing for degradation of an adsorbent material positioned in an intake of the engine by directing fuel vapor to the adsorbent material with a throttle coupled to the engine intake in closed position, and indicating presence or absence of degradation of the adsorbent material based on an air fuel ratio state in an exhaust system of the engine upon opening the throttle. In this way, by saturating an AIS HC trap with fuel vapor during a vehicle key-off condition, and then monitoring an exhaust air fuel ratio as the AIS HC trap is purged, a degradation of the AIS HC trap may be detected.
In one example, a diagnostic routine of the AIS HC trap may be opportunistically carried out during vehicle key-off conditions when the engine is not operated. The engine may be reverse rotated to remove any remaining fuel vapor from the engine intake manifold to the atmosphere via the exhaust passage. Once an exhaust air fuel ratio, as estimated via a heated exhaust gas oxygen (HEGO) sensor, becomes leaner than stoichiometric, indicating absence of fuel vapor in the exhaust passage, the fuel system may be isolated, and fuel vapor may be generated in the fuel tank by operating the fuel pump. In response to the fuel pressure reaching a threshold pressure, the fuel vapor from the fuel system may be routed to the AIS HC trap via a fuel vapor canister. After a threshold time has elapsed since routing of the fuel vapor to the AIS HC trap, it may be inferred that the vapor has been adsorbed by the AIS HC trap. The engine may be cranked unfueled with the intake throttle closed in order to route any remaining, un-adsorbed, vapor from the intake manifold to the atmosphere via the exhaust passage. The intake throttle may then be opened while continuing to spin the engine, unfueled, such that the ambient air flow may be used to purge the AIS HC trap. The fuel vapor from the AIS HC trap may be desorbed and routed to the exhaust passage with the ambient airflow. Flow of desorbed fuel vapor through the exhaust passage may cause the exhaust air fuel ratio to change from leaner than stoichiometric to richer than stoichiometric air fuel ratio. The AIS HC trap may be diagnosed to be degraded responsive to the exhaust air fuel ratio remaining leaner than stoichiometric during the AIS HC trap purge. Upon detection of degradation of the AIS HC trap, upon completion of an immediately subsequent engine operation, the engine may be spun unfueled to route any remaining fuel vapors in the intake system to the exhaust catalyst via the engine cylinders.
In this way, by opportunistically using existing engine components, such as heated exhaust oxygen gas sensor, the need for additional sensors and/or equipment for diagnostics of an AIS HC trap may be reduced or eliminated. By using a fuel pump to generate fuel vapor, diagnostics of the AIS HC trap may be carried out even during engine-off conditions. The technical effect of carrying out the diagnostics of the AIS HC trap during engine-off conditions is that during the diagnostics routine, the HC trap is purged, thereby limiting bleed emissions. Overall, by regularly monitoring the health of the AIS HC trap, emissions quality may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.